Allow panels to open in floating windows

### Feature summary

Allow features like the editor, diff viewer, and terminal to be opened in separate floating windows.

### What problem are you trying to solve?

The desktop app now includes many useful tools, which is great because everything can stay in one place.

But on a multi-monitor setup, all these tools are still inside the same main window. I would like to keep Copilot on one screen and move the editor, diff, or terminal to another screen.

### Proposed solution

Allow panels such as the editor, diff viewer, and terminal to be detached from the main window and opened as independent windows.

Similar to how Photoshop lets you undock panels and move them to other screens.

### Workflow impact

This would make multi-monitor setups much better and reduce the need to switch between panels.

For example:

- Main screen: Copilot
- Second screen: editor or diff
- Third screen: terminal

### Installation context

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points. Begin by locating how the desktop app creates the editor, diff viewer, and terminal panels and how windows are managed; done means those panels can detach into independent windows and move across monitors.

Written by the indexing model from the issue text.